The Celts worshiped Nature, which was also their temple, and they used the properties of many plants to make their ritual potions. In fact, many years ago, the British writer and professor, Robert Graves, wrote an entire classic about the Celts, the Druids and the origins of poetry, among other fascinating background topics, which, titled The White Goddess, already intuited the existence of a Celtic alphabet made up of different types of sacred trees, including, of course, the oak. I also don't understand the obsession that some have with planting foreign species, such as eucalyptus, in these places. Not only is it an invasive species, but it also dries out and spoils the land on which it sits. Ultimately, everything comes down to political and economic interests. But that is another topic.